* {
 box-sizing: border-box;
}

.calc {
  margin: 10px;
  width: 234px;
  border: 1px solid #000;
  border-radius: 10px;
  overflow: hidden;
}

.calc-screen {
  background-color: #292a29;
  height: 80px;
  line-height: 80px;
  padding: 0 10px;
  font-size: 40px;
  text-align: right;
  color: #fff;
}

.calc button {
  background-color: #5e5f5e;
  display: inline-block;
  width: 25%;
  height: 50px;
  line-height: 50px;
  text-align: center;
  font-size: 24px;
  border-color: #000;
  border-style: solid;
  border-width: 0 1px 1px 0;
  color: #fff;
  cursor: pointer;
}

.calc button.zero {
  width: 50%;
}

.calc button.ctrl {
  background-color: #3e3f3e;
}

.calc button.exp {
  background-color: #fd8d0e;
}